This invention relates to thermal batteries employing cathodes comprising a major amount of a chromium (V) compound as cathode active material. In another aspect, the invention relates to a cathode for use in a thermal battery. In still another aspect, the invention relates to the use of chromium (V) compounds as cathode materials in ambient temperature batteries. (NOTE: In discussions of battery technology, the terms cathode, depolarized cathode, ad depolarizer are frequently used. Hereinafter, only the term "cathode" will be used, and will be understood to embrace the meanings carried by any of these terms.)
Thermal batteries are generally described as batteries which have an electrolyte comprising a salt mixture, (i.e., LiCl/KCl eutectic), which is solid at ambient temperatures, and nonconductive to ions. When fused, the salt mixture becomes fluid and ionically conductive. The development of thermal battery technology and details thereof are discussed in more detail in a report published by The American Society of Mechanical Engineers, entitled A Review of Thermal Battery Technology by B. H. Van Domelen and R. D. Wehrle, reprinted from the 9th Intersociety Energy Conversion Engineering Conference, and incorporated by reference herein. More recent developments in thermal battery technology, although directed to a different aspect than the present invention are disclosed in related U.S. Patent Application Ser. No. 361,151, filed Mar. 23, 1982 by James R. Armijo et al., now U.S. Pat. No. 4,383,014 the disclosure therein being incorporated by reference herein.
The disclosure of said patent application relates to an improvement in thermal battery technology wherein an FeS.sub.2 cathode includes an additive for stabilization purposes. However, although an improvement over the prior art, FeS.sub.2 cathodes, including those of the above-discussed application, have a number of disadvantages. For instance, although exhibiting longer life and greater voltage stability than prior art CaCrO.sub.4 cells, FeS.sub.2 cells require a separator pellet or layer between the cathode and the anode resulting in a complicated construction. In addition, the FeS.sub.2 cathode has a limited range or maximum temperature limit because it thermally decomposes in LiCl/KCl eutectic at approximately 550.degree. C.
The Journal of the Electrochemical Society discloses in Vol. 124, page 968, (1977), in an article by J. R. Besenhart et al, that some testing has been done with chromium oxides as cathodes for secondary high energy density lithium batteries. However, there is no discussion or appreciation therein of the use of the specific Cr(V) compounds applied to both thermal batteries and ambient temperature batteries as cathodes in accordance with the present invention.
The common cathode materials known to the inventors to be used in thermal batteries are CaCrO.sub.4, V.sub.2 O.sub.5 and FeS.sub.2. Of these, CaCrO.sub.4 and V.sub.2 O.sub.5 are soluble in molten LiCl/KCl eutectic. Therefore, a portion of the active cathode material must be expended in the formation of a separator layer to prevent self discharge. Cathodes of these materials are also subject to concentration polarization in the course of discharge.
FeS.sub.2 is insoluble in LiCl/KCl eutectic, hence avoiding the two difficulties mentioned above. FeS.sub.2, however, possesses limited thermal stability, undergoing thermal decomposition in LiCl/KCl eutectic at approximately 550.degree. C.
Like FeS.sub.2, the chromium (V) compounds, subject of this disclosure, are insoluble in molten LiCl/KCl eutectic, avoiding the necessity of separator formation and concentration polarization. Unlike FeS.sub.2, these compounds are not subject to decomposition in LiCl/KCl to at least 600.degree. C.